Don't get me wrong, I love the game, but if you complete a level on the hardest difficulty it should also count as having beat it on Easy and Medium.

For a 15 dollars I can already say there's plenty enough content in the game to justify its purchase, taking me about 9 hours total to complete the game (including all costume unlocks and completed retro levels).

I know for a fact that this will be the reason I keep coming back, so at least it has that for a nice hook. May not be in your best interest to grind it out, but it'll always be there for something to do on a rainy day.